Recently, ultrasound contrast agents have been used for therapeutic purposes to enhance the heating effect of High Intensity Focused Ultrasound (HIFU). It has been shown that the heat transfer from the bubble is not negligible for medical ultrasound sound conditions. The heat transfer depends on the Peclet number and the shell properties of the bubble as well as the specific heat ratio of the internal gas. We investigate the heat transfer at various Peclet numbers, with or without a shell, numerically. The results show that there is an optimal Peclet number which corresponds to the maximum heat transfer from the bubble to the surrounding medium, and the viscous shell increases the thermal damping due to the heat generation in the shell.